1. Field of the Invention
The subject matter disclosed generally relates to the field of network servers.
2. Background Information
Computer networks typically include a plurality of client computers that are linked to a number of servers through a communication network. The servers may be coupled to one or more mass storage systems, sometimes referred to as a network attached storage (NAS). The network attached storage is capable of storing a large number of files.
To open a file the client computer sends a file request to the server. If the server cannot satisfy the file request it retrieves the file from the NAS. Mass storage systems typically contain a large number of hard disk or optical drives. Accessing data from a drive is relatively slow. To minimize access time the retrieved file is typically stored in a cache memory of the server. A subsequent request to the server for the file can be satisfied by the server cache.
The size of the cache in each server is relatively limited. Consequently, many file requests result in the accessing of the NAS, which is slow and decreases the useful bandwidth of the network. It is therefore desirable to increase the effective cache of the servers in a network.
In an article entitled “Efficient Cooperative Caching using Hints”, Prasenjit Sarkar and John Hartman, Department of Computer Science, University of Arizona, Tuscon, the authors discuss a concept referred to as cooperative caching. In a cooperative caching scheme a server that receives a file request that it cannot fulfill seeks to have the request satisfied by the cache of another server. The server caches are effectively combined to create one large cache that can be accessed by each server.
The cooperative cache system includes a manager that controls access to the cache. The manager locates files within the cache of a server(s) and controls the transfer of the located files. The manager also insures cache coherency between each server cache. The cache manager resides on a separate server connected to the network. Having a separate server increases the cost and complexity of implementing a cooperative or distributive cache system. Additionally, the manager server may become inoperative thereby eliminating the cooperation between caches. It would be desirable to have a cooperative cache system that did not require a central manager(s). Such a system must also insure cache coherency and a transmission protocol that assures proper receipt of transferred data.